1. Field of the Invention
This invention relates to display system architectures using a device for selectively transforming partially polarized light, more particularly to a retarder stack for transforming at least partially polarized light for input into various optical devices such as electro-optic modulators, magneto-optic modulators or other optical components.
2. Background of the Related Art
Display systems create color in a number of different ways. In a typical cathode ray tube system, the colors are a result of phosphors being excited by an electron gun, causing them to become luminous. The choice of material from which the phosphor is made helps determine which color is viewed when the phosphor is activated. Additionally, they can have more than one gun, where the nature of the electron beam is changed to form a certain color.
Systems that do not use cathode ray tubes, such as spatial light modulator based systems, use other methods. Spatial light modulator based systems include liquid crystal device (LCD) systems, digital micromirror device (DMD) systems, and grating light valve systems, among others. These systems typically project an image from an array of individual elements, each corresponding to a picture element (pixel) dot on the displayed image.
One method in which they create color images is by having one array for each color, typically red, green and blue. The red image is created by activating the appropriate elements on the array to project an image of red (ON elements) and dark or black spots (OFF elements), at the display surface, while doing the same for green and blue. The three images are projected and converged into one image on the display surface, creating the colored image. The mix of red, green and blue to achieve the right colors is achieved by controlling the amount of time each color is ON for each pixel.
Another method is to use one array and have some type of filtering of the source light from which the image is created. This has the advantage over the three device system in that it requires fewer components and is therefore cheaper and can be made more compact. The typical method illuminates the array in a color sequence, relying upon the human eye to integrate the colors into the proper mix, as in the three device system. Each image for each color is projected to the display surface in the same place as the other colors' images, allowing for the same integration effects as in the three device systems.
However, the color filter can be problematic. A typical solution is to use a color wheel divided into thirds, one for each color, and to place it between the light source and the array. As the color wheel spins, each color illuminates the array while the data for that color is being displayed.
This sequence is fixed and cannot be changed easily if events such as channel changes occur, requiring extra processing and adjustments to allow the system to continue to function. The transitions between the colors in sequence require the light to be turned off while the color changes. The amount of time the light is off depends upon the size of the wheel and the rate at which it rotates. This time can even be longer than the amount of time it takes to load data into the array, reducing the system efficiency.
Additionally, color wheels require motors and a controller to operate and with their own bulk contribute to the size and weight of the display system, as well as the cost. Therefore, a solution is needed that provides color sequential systems with a means for filtering light that is smaller, lighter and faster than the present solutions.
The manipulation or transformation of polarized or partially polarized light is essential in a wide variety of optical systems. Especially with the onset of integrated optics and the processing of optical signals, it is necessary to predictably manipulate the polarization of light before that light proceeds to the next stage in an optical system.
Many optical systems require input light be completely or nearly completely polarized and have a known bandwidth and polarization such as input light from lasers or light emitting diodes (LEDs). It is important, however, to be able to selectively transform portions of relatively wide bandwidth light for eventual use in optical systems. For example, it is desirable to be able to selectively shift a certain band of frequencies within a wide band of frequencies comprising white light.
Color display and color filters are examples of optical systems which utilize wide bandwidth light such as white light to function.